﻿<p>An <em>IfcSectionedSpine</em> is a representation of the shape of a three dimensional object composed by a number of
planar cross sections, and a spine curve. The shape is defined between the first element of cross sections and the last
element of the cross sections. A sectioned spine may be used to represent a surface or a solid but the interpolation of
the shape between the cross sections is not defined.</p>
<p>All cross sections have to define areas by a closed profile to allow for the representation of a solid. All cross
sections have to define curves by an open or closed profile to allow for the representation of a surface. The cross
sections are defined by subtypes of <em>IfcProfileDef</em>, where the consecutive profiles may be derived by a
transformation of the start profile or the previous consecutive profile.</p>
<p>The spine curve shall be of type <em>IfcCompositeCurve</em>, each of its segments represented by
<em>IfcCompositeCurveSegment</em> shall correspond to the part between exactly two consecutive cross-sections.</p>
<p>Figure 3 illustrates an example of an <em>IfcSectionedSpine</em>.</p>
<ul>
<li>The <em>SpineCurve</em> is given by an <em>IfcCompositeCurve</em> with two <em>Segments</em>. The
<em>Segments[1]</em> has a <em>ParentCurve</em> of type <em>IfcPolyline</em> and a <em>Transition</em> =
CONTSAMEGRADIENT. The <em>Segments[2]</em> has a <em>ParentCurve</em> of type <em>IfcTrimmedCurve</em> and a
<em>Transition</em> = DISCONTINUOUS.</li>
<li>Each <em>CrossSectionPosition</em> lies at a start or end point of the <em>Segments</em>.</li>
<li>Each <em>CrossSections</em> are inserted by the <em>CrossSectionPositions</em>. The first two cross sections are of
type <em>IfcRectangleProfileDef</em>, the third is of type <em>IfcDerivedProfileDef</em>.</li>
</ul>
<table summary="illustration">
<tr>
<td><img src="../../../figures/ifcsectionedspine-layout1.gif" alt="spine 1" width="400" height="300" border="0"></td>
</tr>
<tr>
<td>
<p class="figure">Figure 3 &mdash; Sectioned spine geometry</p>
</td>
</tr>
</table>
<p>Figure 4 illustrates the final result of the <em>IfcSectionedSpine</em>. The body (shown transparently) is not
fully defined by the exchange definition.</p>
<table summary="illustration">
<tr>
<td><img src="../../../figures/ifcsectionedspine.jpg" alt="render" width="400" height="300" border="0"></td>
</tr>
<tr>
<td>
<p class="figure">Figure 4 &mdash; Sectioned spine result</p>
</td>
</tr>
</table>
<blockquote class="extDef">NOTE&nbsp; Definition according to ISO/CD 10303-42:1992<br>
A sectioned spine is a representation of the shape of a three dimensional object composed of a spine curve and a number
of planar cross sections. The shape is defined between the first element of cross sections and the last element of this
set.<br>
<br>
NOTE&nbsp; A sectioned spine may be used to represent a surface or a solid but the interpolation of the shape between
the cross-sections is not defined. For the representation of a solid all cross-sections are closed curves.</blockquote>
<blockquote class="note">NOTE&nbsp; Entity adapted from <strong>sectioned_spine</strong> defined in ISO
10303-42.</blockquote>
<blockquote class="history">HISTORY&nbsp; New entity in IFC2x.</blockquote>
<p class="spec-head">Informal Propositions:</p>
<ol>
<li>none of the cross sections, after being placed by the cross section positions, shall intersect</li>
<li>none of the cross sections, after being placed by the cross section positions, shall lie in the same plane</li>
<li>the local origin of each cross section position shall lie at the beginning or end of a composite curve
segment.</li>
</ol>